An organization of nonbelievers is threatening legal action against public schools that participate in an evangelical Christian charity that delivers Christmas toys to poor children.

The American Humanist Association, a national advocacy organization with 20,000 members nationwide, sent letters this week to two elementary schools — one in West Columbia, S.C. — after parents complained their children were being asked to collect toys and money for Operation Christmas Child.

Operation Christmas Child is a project of Samaritan’s Purse, an evangelical relief organization founded by Franklin Graham, son of evangelist Billy Graham. Its stated mission is “to follow the example of Christ by helping those in need and proclaiming the hope of the Gospel.”

The toys collected by Operation Christmas Child come with pledges to Christianity for its recipients to sign. Since its founding in 1993, Operation Christmas Child has sent 100 million boxes of toys to poor children.

According to the humanist association, East Point Academy in West Columbia has organized a toy drive and raised funds for the charity for at least three years. A second school, SkyView Academy in Highlands Ranch, Colo., has participated for “several years” and has acknowledged packaging 500 toy boxes in 2012.

Both schools received letters from the AHA informing them their actions are unconstitutional. The same day it received the letter, East Point Academy said it would cut its ties with the charity out of “an abundance of caution because we do not want to expend school financial resources defending a lawsuit.”

Monica Miller, legal counsel for the AHA’s Appignani Humanist Legal Center, said the schools are violating the First Amendment’s guarantee of the separation of church and state and its ban against government endorsement of religion.

“There is also a problem of a lack of transparency,” she said. “One of the parents was not aware of the Christian nature (of Operation Christmas Child) and he let his kids participate because he assumed the school would not promote religion.”

In its letter to the schools, AHA described the toys as “bribes,” and Miller said schoolchildren were also promised incentives for participation, such as pizza parties and free dress days.

Kelly Wells, a spokeswoman for the DeMoss Group, a public relations firm that represents Samaritan’s Purse, said in an email that the organization is aware that public schools participate in its annual toy drive but does not know how many are involved.

“The project is very clear in its marketing material and website that its mission is to demonstrate God’s love in a tangible way to needy children around the world,” Wells wrote. “Therefore, Operation Christmas Child’s goal is to to generate participation in and through local churches and like-minded groups . . . Of course, any person or group is welcome to participate in packing shoebox gifts if they so choose.”

The AHA has not yet heard from the Colorado school, and Miller said the association will pursue a lawsuit if the school does not terminate its participation.